What do all top-selling iPhone games have in common? It’s the controls, stupid.

How do you make a game which will be highly successful in the App Store? It's a question thousands of developers ask themselves daily. Is it the graphics? Is it the level design? Is it the emotions your game engenders? Is it the time-delay before you prompt the user to rate your app? Is it birds and pigs? Is it integrating with Facebook?

What did all these games do brilliantly? The gameplay of Angry Birds is not original. Throwing projectiles at enemies protected by collapsible structures had been done before, in games like Castle Clout and Armor Games's Crush the Castle. Those games were fun, but not mainstream.

It was primarily Angry Birds' intuitive slingshot controls which made the app into the multi-million dollar franchise it is today. The controls were very easy to grasp, and the act of dragging, releasing, and watching the ensuing destruction is satisfying and cathartic.

The core action of Fruit Ninja is a slicing action to chop fruit. The combination of the slicing gesture and corresponding "juicy" effects is also highly satisfying. It's the digital equivalent of popping bubble wrap.

Doodle Jump uses the iPhone's accelerometer as the controls. Tilting the iPhone side-to-side moves the character. It was a clever (and at the time of release, quite novel) use of the device hardware that made the game simple to pick up. If the game had used buttons instead, it would have been much less successful.

Cut the Rope also uses slicing controls, but for a puzzle game. A sign that a level-based game like Angry Birds and Cut the Rope is well-designed is that even replaying the same level again and again is still fun, because performing the core game actions is satiating.

So what do all these game control systems have in common

1. Easy to pick up

2. Satisfying to perform

What doesn't work so well? Games ported from other platforms often use 'virtual joysticks' or buttons.

Players need to feel that their touches are having a direct effect on the action, after all this "direct manipulation" is a key part of Apple's iOS experience. Playing a football game, I don't want to use buttons or joysticks to move the players, I want to feel like I am directly controlling them.

Grand Theft Auto III is one of the best reviewed console games of all time. It has a complex plot and a massive open world. But on iPhone and iPad it just feels fiddly. You have to poke at virtual buttons on the screen. It ranks reasonably well in the App Store, but is easily beaten by games with better controls.

Other games are over-reliant on taps. One-touch controls can work really well (see: Tiny Wings) but other games which follow a "touch everything which moves!" metaphor aren't so interesting.

So: want to make a top-selling iPhone game? Make your controls easy to pick up and satisfying to perform. If you can do that, AND do something unique, you could have a hit. (Of course you'll still need great graphics, music, integration, level design, and so on: but if you don't have great controls, all of those may be in vain)

When developing our new game Flockwork, we think we've created a control system which is easy and satisfying, but also unique.

Everyone quickly understands how to drag their finger on an iPad screen.The twist is that in Flockwork, when you drag, you don't just move one sheep character, but all the sheep at once. This turns a simple puzzle game into something much more interesting - since you can't control the sheep individually, you have to use all the objects in the game environment to help get the sheep to the targets. Watch our trailer below for more:

And if you'd like to sign up to receive an email when Flockwork is launched, visit our Flockwork microsite.